Tamron SP 500mm F/8 Mirror Model 55B: When introduced in
1979, this lens was the smallest and lightest 500mm mirror telephoto ever
produced by any manufacturer. Remember the days of using Vivitar's heavy solid
Catadioptric telephoto lenses or other manufacturer's far less compact mirror
telephotos? This lens is significant because it changed the thinking of optical
engineers throughout the industry since this lens uses Mangin mirrors. Mangin
mirrors are simply lenses with one side of the lens being either aluminized or,
in the case of Tamron's mirror lenses, silvered for peak reflectivity. Light
passes and refracts through the unsilvered front side of the lens element,
reflects off the aluminized or silvered back surface, and then the light again
passes and refracts through the front side of the lens element. The whole point
of using Mangin mirrors within a catadioptric lens is to reduce weight and size
by getting away from the heavy solid catadioptric lens designs and earlier
conventional mirror lens designs which were nowhere as compact.

Many other manufacturers, subsequent to the introduction of this
lens, followed suit by designing cat lenses with very similar optical designs.
For example, a year or two later Tokina introduced their very similar 500mm F/8
which had somewhat worse overall performance. For a while the Tamron and Tokina
lenses were the two best 500mm F/8 mirror lenses on the market, but that
changed after the mid 1980s as some other manufacturers managed to somewhat
improve these landmark optical designs.

Overall, this lens produces fairly sharp images which are slightly
soft in the corners. It is lightweight yet features solid construction, accepts
rear 30.5mm thread-in filters, and produces acceptable performance when used
with the Tamron SP 2x teleconverter. This lens also continuously focuses down
to 1:3 macro. Macro performance is good but with softness in the corners due to
off-axis coma. Light transmission is very close to F/8 and light fall-off is a
mere 0.75 stop. Color rendition is very good. Out-of-focus doughnuts have a
smaller than average "hole" since this lens has a relatively small central
obstruction compared to most other 500mm cat lenses. This is a very good mirror
lens with performance which was only somewhat exceeded by a few of the OEM lens
manufacturers.